Auxiliary power ports are commonly provided in the passenger compartment of automotive vehicles to allow electrical devices such as cigarette lighters, cellular phones, radar detectors, small televisions, and the like to be connected with and receive power from the vehicle electrical system.
The automotive industry has adopted a standard size and configuration for auxiliary power ports used in vehicles. This standard configuration is a hollow, cylindrical receptacle, all or part of the interior surface of which is electrically conductive and connected to the positive or "hot" wire of a direct current circuit, and a terminal disposed at the bottom of the receptacle and connected to electrical ground. The standard adapter plug for mating with such a receptacle has a first terminal that is biased radially outward from the side of the plug to contact the interior surface of the receptacle, and a second terminal at its tip for contacting the ground terminal at the bottom of the receptacle.
The circuit of the vehicle electrical system which supplies electric power to the auxiliary power port is usually protected from overcurrent conditions by a fuse or circuit breaker which is usually located in a power distribution center or fuse block remote from the power port. The fuse or circuit breaker must be rated at a high enough amperage to permit functioning of the highest amperage electrical device which may be inserted into the power port. Consequently, any electrical device with a lower amperage rating will not be protected against overcurrent conditions, but rather may be damaged by levels of current that do not cause the fuse to blow or the circuit breaker to trip.
It is known to provide an adapter plug of an electrical device which houses a conventional cylindrical fuse having an amperage rating appropriate for the particular electrical device. An example of such an adapter plug is disclosed in U.S. Pat. No. 5,199,904. Such an adapter plug, however, may be larger than is desirable in order to house the fuse. Also, once the fuse blows the adapter plug must be partially disassembled and replaced with a spare fuse in order that the device may be used once again.
It is known to protect an electrical circuit from overcurrent conditions by making use of a positive temperature coefficient (PTC) material. Such materials exhibit an electrical resistivity which is relatively low at a design operating temperature band and increases abruptly as the temperature of the material rises beyond a critical temperature. PTC materials include compositions such as conductive polymers and ceramics.
A PTC circuit overcurrent protection device comprises a layer of PTC material sandwiched between two parallel plates of electrically conductive metal. An electrical lead is attached to each of the plates and the leads are connected to the electrical circuit. At a given operating temperature, there is a maximum steady level of electrical current which can pass from one plate to the other through the PTC material without causing significant resistance heating of the device. This level of current is dependent primarily upon the surface area of the layer of PTC material across which the current must flow in passing from one plate to the other, and is known as the "pass" or "hold" current.
Such a PTC device is designed so that when it is subjected to a level of current greater than the hold current, sufficient resistance heating of the device occurs to cause the temperature of the PTC material to climb to above the critical temperature. When this occurs, the electrical resistivity of the PTC layer becomes so great as to create what is essentially an open circuit. A very low level of current continues to pass between the metal plates, however, and this "trickle" of current may be sufficient to prevent the temperature of the device from dropping back below the critical temperature. The circuit must be broken at some other point, for example by switching off an electrical device powered by the circuit, in order for the trickle of current to cease and allow the PTC device to cool down to below its critical temperature so that the PTC material resumes its lower resistivity state. Once this occurs, the PTC circuit overcurrent protection device has essentially reset itself, without the need for any replacement or maintenance of the device, and is again able to provide protection against overcurrent conditions when the electrical device is switched back on.